Method of forming undercut in metal powder injection-molded article

ABSTRACT

To provide a method for forming undercuts of a metal injection molded product by simple processes and metal injection molded products having undercuts formed by the forming method thereof, steps are included of injection-molding a feed stock containing metal powder and binding resin, forming undercuts to an injection green body obtained by the injection-molding, and sintering after debinding the injection green body formed with undercuts. It is preferable that undercuts are formed by deforming at least one part of an injection green body; it is more preferable that they are deformed by pressing with a pressing member.

TECHNICAL FIELD

The present invention relates to a method for forming undercuts of ametal injection molded product and metal injection molded products withundercuts.

TECHNICAL BACKGROUND

In orthodontics, an orthodontic apparatus such as brackets are directlybonded to the teeth normally by the use of adhesive or the like. Forceis applied to the brackets by an orthodontic archwire, and the teeth areforcefully aligned in an appropriate position thereby.

Therefore, various proposals have been made so as to provide sufficientbonding strength to firmly hold an orthodontic apparatus in orthodontictreatment.

As one proposal thereof, undercuts, for instance, are provided on thebonding surface of an orthodontic apparatus to teeth. By providingundercuts, the area of the bonding surface increases, and the bondingstrength increases, since there is an improvement in the holding powerof the dental adhesive.

Methods of providing undercuts to a bonding surface described aboveinclude a method of forming undercuts by machining or the like assecondary work after manufacturing an orthodontic apparatus body, amethod of fixing wire mesh to an orthodontic apparatus body by weldingor the like, and so on.

However, according to the method of carrying out the above-notedsecondary work, as an orthodontic apparatus is generally relativelysmall, there are problems such as extremely complex working, inferiorproductivity and high cost.

Moreover, a strong metal such as titanium is used for orthodonticapparatuses, so that the workability is poor and undercuts are extremelyhard to form. Also, in the method of bonding the wire mesh by welding orthe like, there is the problem that the mesh portion is buried bybrazing filler metal during welding, and the bonding force between thewire mesh and the body gradually decreases.

On the other hand, orthodontic apparatus bodies have been recentlymanufactured by metal injection molding that have high yield, areflexible in molding shapes and can easily mold even complex shapes. Suchan metal injection molding method is used for producing a metal body bydebinding and sintering after injection-molding a feed stock mainly ofmetal powder and binding resin.

With such a metal injection molding method, one-body molding even into acomplex shape is possible, but since a mold is used forinjection-molding, undercuts cannot be directly molded without carryingout the secondary work to the sintered body.

The objective of the present invention is to provide a method forforming undercuts of a metal injection molded product by means of asimple process without the need for secondary work, and to provide metalinjection molded products having undercuts.

DISCLOSURE OF THE INVENTION

(1) A method for forming undercuts of a metal injection molded productaccording to the first invention comprises the steps of:

injection-molding a feed stock containing metal powder and bindingresin;

forming undercuts to an injection green body provided by the injectionmolding; and

sintering the injection green body having undercuts after debinding.

(2) It is preferable that the undercuts are formed by deforming at leastone part of the injection green body.

(3) It is preferable that the undercuts be formed by heating andsoftening the injection green body.

(4) It is preferable that the undercuts be formed by pressing at leastone part of the injection green body with a pressing member.

(5) It is preferable that the pressing surface of the pressing member beheated.

(6) It is preferable that the temperature of the pressing surface be 90°C. to 150° C.

(7) It is preferable that protrusions be formed on the pressing surfaceof the pressing member.

(8) It is preferable that pressing be carried out a plurality of times.

(9) It is preferable that pressing be carried out while changing theshape of the pressing surface.

(10) A method for forming undercuts of a metal injection molded productaccording to a second invention comprises the steps of:

injection-molding a feed stock containing metal powder and bindingresin;

debinding an injection green body-molded by the injection-molding; and

sintering a brown body provided by the debinding; wherein the injectiongreen body is integrated with an undercut molding member in one body.

(11) It is preferable that the undercut molding member comprise aremovable material by heating.

(12) It is preferable that the undercut molding member comprise aremovable material in the debinding step.

(13) It is preferable that the undercut molding member comprise amaterial having resin as a main component.

(14) It is preferable that the resin have a melting point of 150° C. to400° C.

(15) A method for forming undercuts of a metal injection molded productaccording to a third invention comprises the steps of:

injection-molding a feed stock containing metal powder and bindingresin; and

sintering an injection green body molded by the injection-molding afterdebinding;

wherein a mold for injection-molding which has a core for formingundercuts in the mold is used in the injection-molding step.

(16) It is preferable that the core is made up of an elastic material.

(17) It is preferable that the elastic material have a Vickers hardnessof 40 to 100.

(18) It is preferable that the elastic material has tensile strength of60 Kg/cm² or above.

(19) It is preferable that the elastic material consists of siliconerubber.

(20) It is preferable that a metal injection molded product of thepreset invention be manufactured by the methods described in theabove-noted items (1) or (19).

(21) It is preferable that the metal injection molded product be anorthodontic apparatus.

(22) It is preferable that the undercuts be provided to thetooth-bonding surface of the orthodontic apparatus.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a partial sectional view showing a first embodiment of amethod for forming undercuts of a metal injection molded product of thepresent invention.

FIG. 2 is a partial sectional view showing the first embodiment of amethod for forming undercuts of a metal injection molded product of thepresent invention.

FIG. 3 is a partial sectional view showing the first embodiment of amethod for forming undercuts of the metal injection molded product ofthe present invention.

FIG. 4 is a partial sectional view showing a second embodiment of amethod for forming undercuts of the metal injection molded product ofthe present invention.

FIG. 5 is a partial sectional view showing a third embodiment of amethod for forming undercuts of the metal injection molded product ofthe present invention.

FIG. 6 is a view taken on line A—A of FIG. 5.

FIG. 7 is a stereoscopic perspective view of an orthodontic apparatusprepared by the present invention.

FIG. 8 is an enlarged view of the orthodontic apparatus shown in FIG. 7.

FIG. 9 is a cross-sectional view of the orthodontic apparatus shown inFIG. 7.

FIG. 10 is a plane view showing the use of an orthodontic apparatusprepared by the present invention.

EXPLANATION OF REFERENCE NUMERALS

1 orthodontic bracket

2 base

3 engagement part

4 bonding surface

7 slot

8 wire

9 tooth

10 injection green body

11 injection green body

13 protruding part

14 recessed part

15 tapered portion

16 undercut

30 a, 30 b, 30 c pressing member

31 pressing surface

33, 35 protrusion

40 undercut molding member

41 green body

42 brown body

50 core

52 fin

BEST MODES FOR CARRYING OUT THE INVENTION

Methods for forming undercuts of a metal injection molded product of thepresent invention are explained in detail hereafter with reference tothe preferred embodiments shown in the attached figures. A method formanufacturing an orthodontic apparatus (brackets) shown in FIG. 7, as ametal injection molded product with undercuts of the present invention,will be explained as an example.

FIG. 1, FIG. 2 and FIG. 3 are partial sectional views showing a firstembodiment of a method for forming undercuts of the metal injectionmolded product of the present invention.

A method for forming undercuts of a metal injection molded product ofthe present invention comprises the steps of injection-molding a feedstock containing metal powder and binding resin, forming undercuts to aninjection green body obtained by the injection-molding, and sinteringthe injection green body with undercuts after debinding

More specifically, a metal injection molded product may be manufacturedby the steps described below.

First, a feed stock is prepared by mixing metal powder and binding resinby means of a kneading machine. Kneading may be carried out by the useof a kneading machine such as a kneader or a heating blender and thelike.

Kneading conditions are appropriately selected based on variousconditions such as particle diameter and the shape of metal powder, thetype of binding resin, the amount of mixed additives and the like. Ifnecessary, the feed stock may be pelletized.

It is preferable that the ratio of metal powder in the feed stock be inthe range of 80 wt. % to 98 wt. %. By setting the ratio of metal powderin this range, the feed stock has moderate flowability, excellentmolding properties, and can provide a preferable sintered portion.

There is no particular limitation to the metal powder; however, inmanufacturing an orthodontic apparatus or the like, it is preferable touse a dental metallic material such as titanium, titanium alloy andstainless steel. Titanium and titanium alloy are particularly preferablethereof.

Since an orthodontic apparatus is forcefully pulled by a metal wire orthe like, it must have a high level of strength and toughness so as notto be damaged during its use. By using titanium or titanium alloy, theseconditions can be easily satisfied. Furthermore, titanium or titaniumalloy is extremely high in affinity for the human body and isparticularly suitable as a material for an orthodontic apparatus.

Binding resin includes, for instance, polypropylene resin, polyethyleneresin, acrylic resin, polystyrene resin, various kinds of waxes, and soon, and one kind or at least two kinds thereof may be mixed and kneadedfor use. Moreover, in addition to the above-described metal powder andbinding resin, various kinds of additives such as plasticizers,lubricants, antioxidants, debinding accelerators and surface activeagents may be added, depending upon the need.

The feed stock prepared thereby is injection-molded by aninjection-molding machine. Molding conditions of injection-molding arenot particularly limited, and are appropriately set in accordance withthe composition, viscosity, and the like of the feed stock. However, itis preferable that material temperature be about 20° C. to 200° C. andthat the injection pressure be about 30 kgf/cm² to 150 kgf/cm².

As a mold for injection-molding, the same mold as a mold for plasticmolding may be generally used; however, in manufacturing a fine andcomplex green body as, for instance, an orthodontic apparatus, it ispreferable to use a hot runner or the like.

FIG. 1 is a cross-sectional view showing one part of an injection greenbody extracted from a mold for injection-molding.

A plurality of protruding parts 13 are formed in an injection green body10. It is preferable that about sixteen to one thousand six hundredprotruding parts 13 be arranged per 1 cm². The protruding part 13 has atapered portion 15 with a decreasing width along the extractiondirection of the mold.

As these protruding parts 13 of the injection green body 10 are pressedby a pressing member 30 a, tips of the protruding parts 13 are deformedand undercuts 16 are formed. Thus, if an injection green body is not yetsintered, it would be possible to deform at least one part thereof andto easily form the desirable. undercuts.

Forming undercuts by deforming the injection green body 10 arepreferably done by heating and softening. The unsintered injection greenbody 10 contains binding resin and has thermoplasticity, so that it canbe easily deformed by heating.

Methods of heating, softening and then deforming the injection greenbody 10 are not particularly limited, and include a method of contactingthe body with a hot member, a method of adding high energy such asirradiation of high-frequency, a laser beam or the like withoutcontacting, and so on.

In case of this embodiment, the pressing surface 31 of the pressingmember 30 a is heated, and the tips of the protruding parts 13 areheated and softened by contacting them with this pressing surface 31,thus forming undercuts 16 by pressing further.

The temperature of the pressing surface 31 may be optionally selected,but is preferably 90° C. to 150° C. If the temperature of the pressingsurface 31 is too high, the injection green body 10 would be softenedsharply and maintaining the shape would sometimes be difficult.Moreover, decomposition, gasification and the like of the binding resinare likely to occur. On the other hand, when the heating temperature istoo low, the injection green body 10 is sometimes not softened at all,and when pressed in such a condition, a part or a whole body of theinjection green body 10 is broken, cracked and so forth, thus oftendamaging the shape.

FIG. 2 is a cross-sectional view showing another example of moldingundercuts in this embodiment.

As with the above-described case, the injection green body 10 is formedwith a plurality of protruding parts 13. These protruding parts 13 mayor may not be tapered; however, in this embodiment, they are nottapered.

Protrusions 33 are formed on the pressing surface 31 of a pressingmember 30 b so as to match the protruding parts 13.

The protrusions 33 and the protruding parts 13 mutually face each other,and as they are mutually pressed, the protrusions 33 are imbedded in theprotruding parts 13 and tips of the protruding parts 13 are thendeformed and split. These split pieces are pushed outward relative tothe pressing direction, thus forming undercuts 16.

Moreover, the shape, arrangement and so forth of the protrusions 33formed at the pressing surface 31 may be optionally selected and set.Furthermore, as the protruding parts that are formed on the pressingsurface 31, pins or the like may be imbedded.

As in this embodiment, the locations, arrangement and so forth of theundercuts may be arranged by forming the protrusions on the pressingsurface of the pressing member. Therefore, depending upon the purpose,and objective and so forth of the injection molded product, desirableundercuts may be formed.

It is preferable that the pressing surface 31 be heated at 90° C. to150° C. as in the above-mentioned case.

FIG. 3 is a cross-sectional view showing another example of the moldedundercuts in this embodiment.

The same ones as in FIG. 2 may be used for the injection green body 10and the pressing member 30 c.

As shown in the figure, protrusions 35 are formed on the pressingsurface 31 of the pressing member 30c so as to face each recessed part14.

As the tips of the protrusions 35 are pressed so as to face the recessedparts 14, the protrusions 35 are pressed into the recessed parts 14. Asa result, the tips of the protruding parts 13 adjoining recessed parts14 are pushed aside, thereby forming undercuts 16.

It is preferable that the pressing surface 31 be heated at 90° C. to150° C. as in the above-mentioned case.

Moreover, the shape, arrangement and so forth of the protrusions 35formed on the pressing surface 31 may be optionally selected and set.Furthermore, as the protrusions that are formed at the pressing surface31, pins or the like may be imbedded.

Additionally, in case of forming undercuts with a pressing member,pressing may be done once or more.

In pressing several times, pressing may be repeated with the pressingsurface having an identical shape or with a pressing surface of adifferent shape; however, it is preferable to press using pressingsurfaces of different shapes. By the combination of pressing surfaces ofdifferent shapes, various undercuts may be formed.

Then, the injection green body 10 formed with undercuts thereby isdebinded.

Debinding is carried out by means of heat treatment in a non-oxidizingatmosphere, for instance, under a vacuum or in a low-pressure condition(e.g., 1×10⁻¹ Torr to 1×10⁻⁶ Torr) or in an inert gas such as nitrogengas and argon gas, and by the evaporation or decomposition of thebinding resin. Debinding conditions such as temperature increase rate,debinding temperature, and so forth may be set so as not to evaporate ordecompose the binding resin rapidly, and to avoid reactions between thebinding resin and the metal powder. It is preferable that the debindingtemperature be in the range of, for instance, 150° C. to 750° C.

Finally, a brown body is sintered.

It is preferable that sintering be carried out at a low-pressure of1×10⁻² Torr or below (more preferably, 1×10⁻² Torr to 1×10⁻⁶ Torr) or ina vacuum, or in an inert gas atmosphere such as nitrogen gas and argongas at 1 Torr to 760 Torr.

The sintering temperature and sintering time are set so as to diffuseand grow metal powder grains to be crystal grains by sintering, so thatthe powder will be a dense sintered body as a whole, in other words, abody with high density and low porosity.

It is preferable that the sintering temperature be 1000° C. to 1500° C.,and that the sintering time be 0.5 hours to 10 hours.

After sintering, a metal injection molded product with undercuts isprovided.

According to the method of the present invention, metal injection moldedproducts having undercuts may be easily obtained without secondaryworking and with only continuous processes.

Moreover, without requiring a particular injection mechanism or a moldstructure, pressing members (pressing surfaces) are exchanged, so thatit is possible to mold undercuts in a preferable size, shape andarrangement depending upon the purpose.

FIG. 4 is a partial sectional view showing a second embodiment of amethod for forming undercuts of a metal injection molded product of thepresent invention.

The present invention is a method for forming undercuts of a metalinjection molded product comprising the steps of injection-molding afeed stock containing metal powder and binding resin, debinding aninjection green body molded by means of the injection molding, andsintering a brown body obtained by debinding; wherein the injectiongreen body is integrated with an undercut molding member in one body.

Feed stock preparation and injection-molding conditions, and the likeare the same as in the first embodiment. Mostly, the differences fromthe first embodiment are explained hereafter, and items having the sameexplanation will be omitted.

FIG. 4 is a partial sectional view showing a part of an injection greenbody and a brown body in this embodiment.

As shown in this figure, the injection green body 11 is made up of agreen body 41 consisting of a feed stock which contains metal powder andbinding resin, and an undercut molding member 40. The green body 41 andthe undercut molding member 40 are mutually joined together into onebody on the protruding parts 13 of the green body 41.

Such an injection green body 11 may be obtained by injection-molding afeed stock while first filling the undercut molding member 40 into amold for injection-molding as a core.

For materials of the undercut molding member 40, any material isacceptable as long as it is removable and does not damage the shape ofthe green body 41 or the brown body 42, including e.g. any decomposableand removable material by heating, dissolving or other methods. Amongthese, a removable material by heating is preferable, and a removablematerial in the debinding step is further preferable. Debinding thegreen body 41 and removing the undercut molding member 40 may be carriedout simultaneously in one step.

As such a material for the undercut molding member 40, there are noparticular limitations as long as the material can hold a shape evenwith the injection temperature of a feed stock during injection-molding,and is removable along with binding resin in the debinding step. Amaterial containing resin as a main component is preferable, and a resinmaterial having a melting point of 150° C. to 400° C. is morepreferable. Such resins include e.g. polypropylene, polyethylene,polystyrene, polyacetal, acrylic resin, and so forth.

The undercut molding member 40 is removed from the injection green body11 thereby, thus providing the brown body 42 that is molded with theundercuts 16 as shown in the figure.

By sintering the brown body 42 as in the first embodiment, a metalinjection molded product having undercuts may be provided. The undercutmolding member 40 is used as in this embodiment that consists of aremovable material by heating or the like after injection-molding. Thus,there will be no limitations on mold-release from a mold forinjection-molding, flexibility on the shapes and sizes of a green bodywill improve sharply; and any undercuts may be formed.

FIG. 5 and FIG. 6 are partial sectional views showing a third embodimentof the method for forming undercuts of a metal injection molded productof the present invention.

The present invention is a method for forming undercuts of a metalinjection molded product comprising the steps of injection-molding afeed stock containing metal powder and binding resin, and sintering aninjection green body molded by injection-molding after debinding;whereas a mold for injection-molding having a core for forming undercutsin the mold is used in the injection-molding step.

The same feed stock preparation, injection-molding conditions, and soforth may be carried out as in the first embodiment and the secondembodiment mentioned above.

Mostly, differences between the first embodiment and the secondembodiment will be explained, and explanations which are the same willbe omitted. FIG. 5 is a partial sectional view showing one example of acore provided in a mold for injection-molding used in this embodiment.FIG. 6 is a view taken on line A—A of FIG. 5.

The core 50 is formed with a fin 52 as shown in the figure. The fin 52has its tips split four ways, and an injection green body is formed withundercuts by the shape of this fin 52.

It is preferable that such a core 50 be made up of an elastic material.The fin 52 can be stretched or deformed, and may be easily released fromthe undercuts 16 of the injection green body 10.

The fin 52 has open tips as shown in FIG. 5 and FIG. 6. When the fin 52maintains this shape after injection-molding, it engages in the releaseedirection of the core 50, so that release from the injection green body10 will be extremely difficult. However, when the fin 52 has elasticityas in the present invention, the tips of the fin 52 close as the core 50is being released, so that releasing becomes possible.

By fully utilizing characteristics such as flexibility and deformationof an elastic material as described above, it is possible to easilyremove, from a mold, an injection green body with undercuts which couldnot be removed solely by opening and closing the mold. As the elasticmaterial constituting the core 50, material having a Vickers hardness of40 to 100 is, for instance, preferable. With below 40 in hardness,sufficient rigidity and shape maintaining properties cannot be obtainedas a core; and at above 100, the fin 52 cannot be fully deformed,causing problems such as scratching the injection green body 10 duringextraction.

Moreover, the tensile strength is preferably 60 Kg/cm² or above.

At less than 60 Kg/cm² of tensile strength, the strength will not besufficient to withstand stress during extraction, and extraction will besometimes difficult.

Materials of such a core 50 include EVA resin (ethylene-vinyl acetatecopolymer), chlorinated polyethylene, thermoplastic elastomer, rubbermaterial (natural rubber, synthetic rubber), and the like. Examples ofsynthetic rubber include e.g. polyisoprene rubber, butadiene rubber,1,2-polybutadiene rubber, styrene-butadiene rubber, chloroprene rubber,nitrile rubber, butyl rubber, ethylene-propylene rubber,chlorosulfonated polyethylene, acrylic rubber, epichlorohydrine rubber,silicone rubber, fluoro-rubber, urethane rubber, and the like. Theserubber materials may be used solely or as a mixture of two or morekinds; however, among these, silicone rubber is preferable. Siliconerubber is applicable in a wide temperature range and is excellent inheat resistance, oil resistance and weather resistance. Thus, it is nottransformed, decomposed, etc. by temperature and pressure duringinjection-molding, and the shape of the fin 52 may be kept well.Moreover, it is unlikely that the rubber will react to feed stockcomponents such as metal powder.

The core 50 may be entirely made up of an elastic material like siliconerubber in the present invention, or may be made up of an elasticmaterial, a metal material, and so forth. In this case, it is preferablethat at least fin 52 be made up of an elastic material such as siliconerubber and that other parts be made up of a metal material or the like.

After the core 50 is removed, a metal injection molded product havingundercuts may be obtained by debinding and sintering the injection greenbody 10 as in the first embodiment.

The undercuts formed as in the above-mentioned embodiment may have aconstant or irregular pattern. Also, undercuts may be formed so as tokeep the angles and sizes thereof the same, or to change angles and soforth irregularly based on a predetermined order.

Moreover, a metal injection molded product with undercuts that is moldedin the present invention may be for any use, but includes, for example,an orthodontic apparatus.

In the case of an orthodontic apparatus, as shown in FIG. 7 and FIG. 9,it is preferable to form undercuts 16 to a bonding surface 6 to teeth.As a result, a surface area of the bonding surface 6 will increase, andadhesion will be fully maintained, providing excellent adhesivestrength.

Methods for forming undercuts of metal injection molded products havingundercuts of the present invention were explained above by referring toeach embodiment shown in figures, but the present invention is notlimited to these. For instance, as an undercut molding member, resinousbeads or the like may be used.

Embodiments

Next, specific embodiments of the present invention will be explained.

Embodiment 1

Titanium powder (average particle size: 15 μm, 92 wt. %), polypropyleneas binding resin, ethylene-vinyl acetate copolymer(EVA), and othervarious additives (paraffin wax, dibutylphthalate) were mixed at apredetermined ratio, and were kneaded by a kneading machine, thuspreparing a feed stock.

Then, this feed stock was heated to melt the binding resin and wasinjected into a mold for injection-molding, thus providing an injectiongreen body 10.

The tips of protruding portions 13 of this injection green body 10 werepressed with a pressing member 30 a having a pressing surface 31 withthe shape shown in FIG. 1 and heated at 120° C., thus forming undercuts16.

The injection green body 10 formed with undercuts 16 was heated for onehour at 450° C. in vacuum by a predetermined debinding furnace,decomposing and removing the binding resin and the additives withoutdamaging the shape of the green body and thus providing a brown body.

Then, the brown body was shifted to a sintering furnace, and wassintered for three hours at 1,200° C. in argon gas and was then cooled,providing an orthodontic bracket 1 formed with undercuts 16 at thebonding surface 6 as shown in FIG. 7 or FIG. 9.

Each part of this orthodontic bracket 1 has the following shape:

Size of base 2: 5 mm×5 mm

Protruding parts 13: 100 per 1 cm²

Depth of recessed part of an undercut 16: 0.3 mm

Depth of a slot 7: 2.5 mm, Width thereof: 1.3 mm

Embodiment 2

A feed stock having the same composition as in Embodiment 1 wasprepared.

Then, an undercut molding member 40 as shown in the figure (made ofpolystyrene; melting point of 200° C.) was provided inside a mold forinjection-molding. A feed stock was injected into this mold forinjection-molding as in Embodiment 1, thus providing an injection greenbody 11 as shown in FIG. 4.

This injection green body 11 was heated for 1.5 hours in nitrogen gas bya predetermined debinding furnace; the binding resin and the additiveswere decomposed and removed; and at the same time, the undercut moldingmember 40 was removed, thus providing a brown body 42 formed withundercuts 16.

Then, this brown body 42 was shifted to a sintering furnace, and wassintered for three. hours at 1,200° C. in vacuum and was then cooled,providing the orthodontic bracket 1 as in Embodiment 1.

Embodiment 3

A feed stock having the same composition as in Embodiment 1 wasprepared.

Then, this feed stock was heated to melt the binding resin, and wasinjected into a mold for injection-molding. In the mold forinjection-molding, a core 50 made of silicone resin having a shape asshown in FIG. 5 was provided in advance.

After the injected feed stock was solidified, the mold forinjection-molding was opened and the core 50 was pulled out from theinjection green body. The tips of the fin 52 were closed as the core 50was pulled out of the injection green body, so that it was possible toextract the core without damaging the undercut shape of the injectiongreen body.

The injection green body formed with the undercuts thereby was debindedand sintered as in Embodiment 1, thus providing the same orthodonticbracket 1 as in Embodiment 1.

Dental adhesive was coated onto the bonding surface 6 of eachorthodontic bracket 1 prepared in Embodiments 1 to 3, and was fixed tothe surface of a tooth 9 as shown in FIG. 10. Then, a stainless wire 8was passed through slots 7 provided at engagement parts 3 of brackets 1,and was mounted.

One end of the wire 8 was fixed in this condition and sufficient tensionwas added by pulling the other end. The brackets 1 were preferably fixedto the teeth 9, and there appeared to be no shaky, separated or so forthbrackets 1.

As described above, according to the method of the present invention, ametal injection molded product having undercuts can be provided withoutrequiring unique mold structures or the like, and without requiringcomplex secondary processing. Furthermore, undercut patterns and thelike may be easily arranged and changed.

When a metal injection molded product with undercuts is an orthodonticapparatus, a bonding area increases due to the formation of undercutsand bonding strength to teeth may be maintained and improved.Furthermore, by selection of angles, locations and the like ofundercuts, bonding strength may be preferably arranged.

INDUSTRIAL APPLICATIONS

Metal injection molded products of the present invention are suitablefor, for instance, an orthodontic apparatus.

What is claimed is:
 1. A method for forming undercuts of a metalinjection molded product comprising the steps of: injection-molding afeed stock containing metal powder and binding resin; forming undercutsto an injection green body provided by the injection molding wherein theundercuts are formed by pressing at least one part of the injectiongreen body with a pressing member having a heated pressing surface; andsintering the injection green body formed with undercuts afterdebinding.
 2. The method of claim 1, wherein the undercuts are formed byheating and softening the injection green body.
 3. The method of claim1, wherein temperature of the pressing surface is 90° C. to 150° C.
 4. Amethod for forming undercuts of a metal injection molded productcomprising the steps of: injection-molding a feed stock containing metalpowder and binding resin; forming undercuts to an injection green bodyprovided by the injection molding, wherein the undercuts are formed bypressing at least one part of the injection green body with a pressingmember, wherein protrusions are formed at a pressing surface of thepressing member.
 5. The method of claim 1, wherein the pressing iscarried out a plurality of times.
 6. The method of claim 5, wherein thepressing is carried out while changing a shape of the pressing surface.7. A method for forming undercuts of a metal injection molded productcomprising the steps of: injection-molding a feed stock containing metalpowder and binding resin; and sintering an injection green body moldedby the injection-molding after debinding; wherein a mold for injectionmolding having a core made up of an elastic material for formingundercuts in the mold is used in the injection-molding step.
 8. Themethod of claim 7, wherein the elastic material has a Vickers hardnessof 40 to
 100. 9. The method of claim 7, wherein the elastic material hastensile strength of 60 kg/cm² or above.
 10. The method of claim 7,wherein the elastic material consists of silicone rubber.
 11. The methodof claim 4, wherein the undercuts are formed by heating the pressingmember and softening the injection green body with the pressing member.12. The method of claim 4, wherein the pressing is carried out aplurality of times.
 13. The method of claim 12, wherein the pressing iscarried out while changing a shape of the pressing surface.
 14. A methodfor forming undercuts of a metal injection molded product comprising thesteps of: injection-molding a feed stock containing metal powder andbinding resin; forming undercuts to an injection green body provided bythe injection molding, wherein the undercuts are formed by heating andsoftening the injection green body; and sintering the injection greenbody formed with undercuts after debinding.
 15. The method of claim 14,wherein said step of heating and softening the injection green bodyfurther comprises pressing at least one part of the injection green bodywith a pressing member.
 16. The method of claim 15, wherein a pressingsurface of the pressing member is heated.
 17. The method of claim 16,wherein temperature of the pressing surface is 90° C. to 150° C.
 18. Themethod of claim 15, wherein protrusions are formed at a pressing surfaceof the pressing member.
 19. The method of claim 18, wherein the pressingis carried out a plurality of times.
 20. The method of claim 19, whereinthe pressing is carried out while changing a shape of the pressingsurface.